This invention relates to optical communications systems and in particular to optical communications systems including optical amplifiers.
Optical communications systems in which data is transmitted between transmitter and receiver terminals over optical links, in particular optical fibre links, may employ optical amplifiers comprised by semiconductor devices or erbium doped silica fibres, for example, in the optical links. The use of an optical amplifier enhances the sensitivity of the optical receiver at the receiver terminal. The degree of enhancement depends on the amount of optical filtering used between the optical amplifier (also known as an optical pre-amplifier) and the receiver. In principle, the narrower the filter, the higher the sensitivity enhancement. However, the use of narrow band optical filters has two disadvantages. The first is the requirements it imposes on the selection of the transmitter laser wavelength and the second is the requirement it imposes on the stability of the transmitted wavelength with time over the lifetime of the system.
The present invention aims to remove both of these restrictions.